Plasma etching of Cu-containing layers

ABSTRACT

A method and apparatus are provided for plasma etching of Cu-containing layers in semiconductor devices using an aluminum source in the presence of a halogen-containing plasma. The aluminum source reacts with halogenated Cu-containing surfaces and forms volatile etch products that allows for anisotropic etching of Cu-containing layers using conventional plasma etching tools.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority and is related to U.S.application Ser. No. 60/379,754, filed on May 14, 2002. The presentapplication is also related to U.S. application Ser. No. 60/392,045,filed on Jun. 28, 2002. The entire contents of both of thoseapplications are herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to plasma etching of copper-containinglayers used in semiconductor applications.

BACKGROUND OF THE INVENTION

Copper (Cu) is emerging as the metal of choice in a wide variety ofsemiconductor applications. Lower electrical resistivity, coupled withimproved electromigration performance and increased stress migrationresistance are important material properties that favor the use of Cuover aluminum (Al) in interconnect lines and contacts. The lowerelectrical resistance is critical since it allows signals to move fasterby reducing the RC time delay. The superior resistance toelectromigration, a common reliability problem in Al lines, means thatCu can handle higher power densities. An equally important benefit of Cuover Al is that the manufacturing cost for a Cu metallization scheme canbe lower due to new processing methods that reduce the number ofmanufacturing steps and alleviate the need for some of the mostdifficult steps.

However, the introduction of Cu into multilevel metallizationarchitecture requires new processing methods for Cu patterning. BecauseCu is difficult to dry etch, new process schemes have been developed forCu patterning. The damascene approach is based on etching features inthe dielectric material, filling them with Cu metal, and planarizing thetop surface by chemical mechanical polishing (CMP). Dual damasceneschemes integrate both the contacts and the interconnect lines into asingle processing scheme. However, Cu CMP technology is challenging andit has difficulty defining extremely fine features.

An alternative to the damascene approach is a patterned etching of a Culayer. The patterned etch process involves deposition of a Cu layer on asubstrate; the use of a patterned hard mask or photoresist over the Culayer; patterned etching of the Cu layer using a reactive ion etching(RIE) process; and deposition of dielectric material over the patternedCu layer. Patterned etching of Cu can have advantages over damasceneprocesses since it is easier to etch fine Cu patterns and then deposit adielectric layer onto the Cu pattern, than it is to get barrier layermaterials and Cu metal to adequately fill small feature openings in adielectric film.

Magnetoresistive random access memory (MRAM), is an example of newmemory technology that can benefit from new dry etching methods forpatterning Cu layers with magnetic materials. The lack of suitable fastetching processes for materials such as Cu and various magneticmaterials, can limit the ability to batch-fabricate sub-micron magneticdevices. Magnetic materials, that contain transition metals such as Ni,Fe and Co, are substantially inert in conventional dry etch processes.Patterning of magnetic devices has been predominantly accomplished usingAr⁺-ion milling or additive deposition processes such as electroplatingor lift-off, but these methods are undesirable for semiconductor batchprocessing.

The primary etch gas for etching Al and Cu layers is traditionally achlorine-containing gas in a gas mixture that includes argon (Ar). Thechlorine-containing gas is selected from a large group of chlorinecompounds such as Cl₂, HCl, BCl₃, SiCl₄, CHCl₃, CCl₄, and combinationsthereof. To achieve anisotropic etching, Cl₂ is mixed with otherchlorine-containing gases that are selected from the above list, sincethe use of Cl₂ alone results in isotropic etching.

Etching of Cu layers using chlorine plasma essentially involves physicalsputtering of the CuCl_(x) layer by energetic ions in the plasma. Theetching rates with this method are very low and another drawback is thatthe sputtered CUCl_(x) coats the chamber walls and this requiresperiodic cleaning of the chamber. An equally serious problem isencountered when high-aspect-ratio features are etched in chlorineplasma and the sputtered CuCl_(x) products redeposit on the featuresidewalls where the effects of physical sputtering are reduced.

Furthermore, when the process is carried out at elevated temperatures(>200° C.) to increase the volatility of the reacted Cu layer, corrosioncan occur due to accumulated CuCl_(x) etch residues on the surface. Ifthese residues are not removed by a post-etch cleaning step, they cancause continuing corrosion of the Cu even after the application of aprotective layer over the etched features.

Other approaches for dry etching of Cu that involve copper halides havebeen examined to try to accomplish higher Cu etch rates. In addition tohigh processing temperature, the use of additional energy sources, suchas exposure of the etch surface to UV or IR light to accelerate thedesorption of CuCl_(x) have been proposed. These alternative approachesare not practical for semiconductor batch processing of large substratesdue to poor etch uniformity, high cost and added equipment complexity,and reliability problems.

Nelson in U.S. Pat. No. 4,468,284 entitled “Process for etching analuminum-copper alloy,” describes a process for etching Al—Cu alloysthat contain up to 6% Cu by weight. The plasma process comprisesreactive chlorine and a NO⁺ species that aids in the oxidation of Cu toCuCl₂. An Al₂Cl₆ reactant is formed in-situ from the etching of Al inthe Al—Cu alloy and it reacts (complexes) with the surface CuCl₂ to forma volatile CuCl₂—Al₂Cl₆ complex that is removed from the etchingsurface.

Bausmith et al. in U.S. Pat. No. 4,919,750 entitled “Etching metal filmswith complexing plasma,” describes a method for dry etching metals thatform low-volatility chlorides. The method involves exposing a layer ofmetal to a chlorine-based plasma in presence of a metal source spacedapart from the workpiece. When reacted with the plasma, the metal sourceprovides gaseous metal-containing reactants that serve as complexingagents with the metals in the surface of the workpiece to be etched. Theworkpiece can comprise Co, Cu or Ni metal films and the metal source isselected from Al, Ga, Fe and In.

Both of the abovementioned patents involve forming etching reactantsin-situ during the process, which can result in poor control over thedelivery of the etching reactant to the surface of the metal film to beetched. Therefore, to provide better etch control and better controlover reactant delivery; it is desirable to introduce gaseous etchingreactants from ex-situ sources such as gas cylinders or precursorcontainers.

Due to the introduction of Cu in new and existing thin-filmtechnologies, there is a need for dry etching methods that allow etchingand patterning of pure Cu layers and Cu-containing layers, using gaseousreactants that form volatile Cu-containing etch products.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide a plasmaprocessing system and method for etching pure Cu films and Cu-containingfilms for manufacturing devices on a substrate (e.g., a liquid crystaldisplay on a liquid crystal display panel or an integrated circuit on asemiconductor wafer). The process environment comprises process gasesthat are capable of anisotropic etching of high-aspect-ratio features inaccordance with a mask pattern.

The above and other objects are achieved, according to the presentinvention, by providing an apparatus and a method that uses a gaseousplasma environment comprising a reactive halogen species and an aluminumsource. Reactive halogen species from the plasma oxidize the surface ofthe Cu-containing layer to be etched, forming low-volatility Cu-halideproducts that further react with the aluminum source to formCu-containing etch products that are volatile at low substratetemperatures and are removed from the substrate with the aid ofion-assisted etching.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will become readily apparent with reference to thefollowing detailed description, particularly when considered inconjunction with the accompanying drawings, in which:

FIG. 1 is a flowchart for etching a Cu layer according the presentinvention;

FIG. 2 shows a plasma processing system according to a preferredembodiment of the present invention;

FIG. 3 shows a plasma processing system according to an alternateembodiment of the present invention;

FIG. 4 shows a plasma processing system according to an alternateembodiment of the present invention;

FIG. 5 shows a plasma processing system according to an alternateembodiment of the present invention; and

FIG. 6 shows a plasma processing system according to an alternateembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In general, the present invention pertains to a plasma processing methodfor etching pure Cu layers, Al—Cu alloys, and other Cu-based alloys formanufacturing an integrated circuit.

The method uses a plasma process that comprises a reactive halogenspecies and an aluminum source to etch Cu-containing layers by formingvolatile reaction products that are removed by ion-assisted etching.

For example, the reactive halogen species can be formed from achlorine-containing gas that is selected from the group containing Cl₂,HCl, BCl₃, SiCl₄, CHCl₃, CCl₄, and mixtures thereof. Reactive halogenspecies can also be formed from non-chlorine containing gases such asaluminum bromide.

Al layers used in integrated circuits commonly contain a small amount ofCu (<5 at. %) to improve the materials properties of the Al layers. Ithas been observed that the addition of Cu to Al layers affects the etchrate of the layers, and this effect has been attributed to the formationof volatile etch products such as CuCl₂—Al₂Cl₆. A reduced etch rate forAl—Cu layer with high Cu content is possibly due to a limited supply ofaluminum chloride from the etching of Al in the Al—Cu film, that isavailable for reacting (complexing) with the CuCl_(x) etch products thathave low volatility and, in the absence of a second etching agent,accumulate on the surface and prevent further etching.

According to the present invention, the aluminum species reacts(complexes) with the halogenated Cu layer and forms etch products thatcontain Al, Cu, and a halogen. In the case of chlorine, Cl-containingetch products are formed, possibly CuCl₂—Al₂Cl₆ and CuCl₂—AlCl₃complexes, that are volatile enough to allow ion-assisted etching atrelatively low substrate temperatures. The invention further allows forpatterning of high-aspect-ratio Cu-containing structures according to ahard mask or a photoresist pattern using conventional plasma tools.

In an alternate embodiment, an inert gas is added to any one of theaforementioned process gas chemistries. The inert gas may include atleast one of argon, helium, xenon, krypton, and nitrogen. For example,the addition of inert gas to the process chemistry is used to dilute theprocess gas or adjust the process gas partial pressure(s). Furthermore,for example, the addition of inert gas can aid the physical component ofthe feature etch.

FIG. 1 is a flowchart for etching a Cu-containing layer according thepresent invention. Step 100 provides a surface having a Cu-containinglayer to a plasma process chamber. In step 102, a process gas comprisinga halogen-containing reactant is introduced to the process chamber, andplasma is formed. The plasma species react with the Cu-containing layerin step 104. During the plasma etching process, a corrosive halidespecies is generated in the plasma and the surface is under constant ionbombardment in addition to exposure to an aluminum species. In step 106,an aluminum source reacts with the halogenated layer to form volatilereaction products that comprise Cu, Al, and a halogen. The volatilereaction products formed in step 106 are removed from the surface instep 108 with aid of ion-assisted desorption. The Cu-containing layer isexposed to the plasma for a time period that enables the desired etchingof the Cu-containing layer.

Various aluminum sources can be used in the present invention to etchCu-containing layers. For example, an aluminum halide compound such asaluminum chloride is a stable gas that is commercially available and hasa relatively high vapor pressure (˜10 Torr at 124° C.) that allows forefficient delivery of the aluminum chloride gas to the etching chamber.Amine alanes are well known aluminum compounds that are used in lowtemperature chemical vapor deposition (CVD) of aluminum. These compoundshave the general formula H₃AlNR₃, where R is an alkyl group. The weakAl—N bond and thermal stability of the amine group (NR₃) allows Aldeposition at temperatures as low as about 60° C. with very low carbonor oxygen contamination. Examples of amine alanes include trimethylaminealane (TMM) and dimethylethylamine alane (DMEAA). Other aluminum sourcesinclude trialkyl aluminum compounds such as trimethylaluminum (TMA),dialkyl aluminum hydrides such as dimethyl aluminum hydride (DMAH), andsputtered aluminum.

The handling and use of the abovementioned reagents are well known inthe semiconductor industry. Liquid reagents can be introduced into theprocessing chamber using a delivery system that can comprise a bubblersystem and a mass flow controller (MFC). A bubbler system can be usedwith or without a carrier gas such as argon (Ar). When a carrier gas isused, it is bubbled through the liquid and becomes saturated withreagent vapor. The partial pressure of the vapor in the process chamberis controlled by the temperature of the liquid in the bubbler.Alternatively, a liquid injection system can be used to deliver thereagents to the processing chamber. In the case of solid reagents, acarrier gas can be passed over the solid and the gas mixture introducedinto the process chamber.

The gas flow rates of the etching gases can be independently controlledduring processing. Exemplary flow rates are from 0 to 1000 sccm, withtypical values being less than 500 sccm and preferably between 1 and 500sccm. It should be noted that an aluminum halide gas may react with acopper containing layer when the gas is in either a plasma form or in agaseous (non-plasma) form.

FIG. 2 shows a plasma processing system according to a preferredembodiment of the present invention. In FIGS. 2-6, like referencenumbers are used to indicate like elements throughout. A plasmaprocessing system 1 that is capable of sustaining a plasma is depictedin FIG. 2, which includes a plasma process chamber 10 configured tofacilitate the generation of plasma in processing region 45, whereinplasma is formed via collisions between heated electrons and anionizable gas. The plasma processing system 1 further comprises asubstrate holder 20, upon which a substrate 25 to be processed isaffixed, and a gas injection system 40 for introducing process gases 42to the plasma process chamber 10. Alternately, gas injection system 40can comprise an auxiliary gas dispenser 44 for introducing processgases. The gas injection system 40 allows independent control over thedelivery of process gases to the process chamber from ex-situ gassources. The process gases can comprise an aluminum source,halogen-containing gases, and inert gases.

Process conditions that enable the desired etching of the Cu-containinglayer in the current invention may be determined by directexperimentation and/or design of experiments (DOE). For example,adjustable process parameters can comprise plasma power, plasmafrequencies, substrate temperature, process pressure, choice of processgases and relative gas flows of the process gases. Plasma power andplasma frequencies are process parameters that can be used to controlthe extent of dissociation of the etching reagents in the plasma, whichin turn affect the efficiency of the etching process. In addition to theabovementioned process parameters, different methods for introducingprocess gases, such as the use of auxiliary gas dispenser 44, allow foradditional control over the extent of dissociation of the etchingreagents.

FIG. 3 shows a plasma processing system according to an alternateembodiment of the present invention. A plasma processing device 1 isdepicted which includes a chamber 10, a substrate holder 20, upon whicha substrate 25 to be processed is affixed, a gas injection system 40,and a vacuum pumping system 50. Chamber 10 is configured to facilitatethe generation of plasma in a processing region 45 adjacent a surface ofsubstrate 25, wherein plasma is formed via collisions between heatedelectrons and an ionizable gas. An ionizable gas or mixture of gases isintroduced via the gas injection system 40 and the process pressure isadjusted. For example, a gate valve (not shown) is used to throttle thevacuum pumping system 50. Desirably, plasma is utilized to creatematerials specific to a pre-determined materials process, and to aideither the deposition of material to a substrate 25 or the removal ofmaterial from the exposed surfaces of the substrate 25.

Substrate 25 is transferred into and out of chamber 10 through a slotvalve (not shown) and chamber feed-through (not shown) via roboticsubstrate transfer system where it is received by substrate lift pins(not shown) housed within substrate holder 20 and mechanicallytranslated by devices housed therein. Once the substrate 25 is receivedfrom the substrate transfer system, it is lowered to an upper surface ofthe substrate holder 20.

In an alternate embodiment, the substrate 25 is affixed to the substrateholder 20 via an electrostatic clamp (not shown). Furthermore, thesubstrate holder 20 further includes a cooling system including are-circulating coolant flow that receives heat from the substrate holder20 and transfers heat to a heat exchanger system (not shown), or whenheating, transfers heat from the heat exchanger system. Moreover, gasmay be delivered to the backside of the substrate to improve the gas-gapthermal conductance between the substrate 25 and the substrate holder20. Such a system is utilized when temperature control of the substrateis required at elevated or reduced temperatures. For example,temperature control of the substrate may be useful at temperatures inexcess of the steady-state temperature achieved due to a balance of theheat flux delivered to the substrate 25 from the plasma and the heatflux removed from substrate 25 by conduction to the substrate holder 20.In other embodiments, heating elements, such as resistive heatingelements, or thermoelectric heaters/coolers are included.

In the embodiment, shown in FIG. 3, the substrate holder 20 furtherserves as an electrode through which radio frequency (RF) power iscoupled to plasma in the processing region 45. For example, thesubstrate holder 20 is electrically biased at a RF voltage via thetransmission of RF power from an RF generator 30 through an impedancematch network 32 to the substrate holder 20. The RF bias serves to heatelectrons and, thereby, form and maintain plasma. In this configuration,the system operates as a RIE reactor, wherein the chamber and upper gasinjection electrode serve as ground surfaces. A typical frequency forthe RF bias ranges from 1 MHz to 100 MHz and is preferably 13.56 MHz.

In an alternate embodiment, RF power is applied to the substrate holderelectrode at multiple frequencies. Furthermore, the impedance matchnetwork 32 serves to maximize the transfer of RF power to plasma inprocessing chamber 10 by minimizing the reflected power. Match networktopologies (e.g., L-type, π-type, T-type) and automatic control methodsare known in the art.

With continuing reference to FIG. 3, a process gas 42 is introduced tothe processing region 45 through the gas injection system 40. Gasinjection system 40 can include a showerhead, wherein the process gas 42is supplied from a gas delivery system (not shown) to the processingregion 45 through a gas injection plenum (not shown), a series of baffleplates (not shown) and a multi-orifice showerhead gas injection plate(not shown). Alternately, gas injection system 40 can comprise anauxiliary gas dispenser 44 for introducing additional process gases.

Vacuum pump system 50 preferably includes a turbo-molecular vacuum pump(TMP) capable of a pumping speed up to 5000 liters per second (andgreater) and a gate valve for throttling the chamber pressure. Inconventional plasma processing devices utilized for dry plasma etch, a1000 to 3000 liter per second TMP is employed. TMPs are useful for lowpressure processing, typically less than 50 mTorr. At higher pressures,the TMP pumping speed falls off dramatically. For high pressureprocessing (i.e. greater than 100 mTorr), a mechanical booster pump anddry roughing pump are used.

A controller 55 includes a microprocessor, a memory, and a digital I/Oport capable of generating control voltages sufficient to communicateand activate inputs to the plasma processing system 1 as well as monitoroutputs from the plasma processing system 1. Moreover, the controller 55is coupled to and exchanges information with the RF generator 30, theimpedance match network 32, the gas injection system 40, plasma monitorsystem 57, and the vacuum pump system 50. A program stored in the memoryis utilized to control the aforementioned components of a plasmaprocessing system 1 according to a stored process recipe. One example ofcontroller 55 is a digital signal processor (DSP), model number TMS320,available from Texas Instruments, Dallas, Tex.

The plasma monitor system 57 can comprise, for example, an opticalemission spectroscopy (OES) system to measure excited particles in theplasma environment and a plasma diagnostic system, such as a Langmuirprobe, for measuring plasma density. The plasma monitor system 57 can beused with controller 55 to determine the status of the etching processand provide feedback to ensure process compliance.

FIG. 4 shows a plasma processing system according to an alternateembodiment of the present invention. The plasma processing system 1 ofFIG. 3 further includes either a mechanically or electrically rotatingDC magnetic field system 60, in order to potentially increase plasmadensity and/or improve plasma processing uniformity, in addition tothose components described with reference to FIG. 3. Moreover, thecontroller 55 is coupled to the rotating magnetic field system 60 inorder to regulate the speed of rotation and field strength.

FIG. 5 shows a plasma processing system according to an alternateembodiment of the present invention. The plasma processing system 1 ofFIG. 3 further includes an upper plate electrode 70 to which RF power iscoupled from an RF generator 72 through an impedance match network 74. Atypical frequency for the application of RF power to the upper electroderanges from 10 MHz to 200 MHz and is preferably 60 MHz. Additionally, atypical frequency for the application of power to the lower electroderanges from 0.1 MHz to 30 MHz and is preferably 2 MHz. Moreover, thecontroller 55 is coupled to the RF generator 72 and the impedance matchnetwork 74 in order to control the application of RF power to the upperelectrode 70.

FIG. 6 shows a plasma processing system according to an alternateembodiment of the present invention. The plasma processing system ofFIG. 2 is modified to further include an inductive coil 80 to which RFpower is coupled via an RF generator 82 through an impedance matchnetwork 84. RF power is inductively coupled from the inductive coil 80through a dielectric window (not shown) to the plasma-processing region45. A typical frequency for the application of RF power to the inductivecoil 80 ranges from 10 MHz to 100 MHz and is preferably 13.56 MHz.Similarly, a typical frequency for the application of power to the chuckelectrode ranges from 0.1 MHz to 30 MHz and is preferably 13.56 MHz. Inaddition, a slotted Faraday shield (not shown) is employed to reducecapacitive coupling between the inductive coil 80 and plasma. Moreover,the controller 55 is coupled to the RF generator 82 and the impedancematch network 84 in order to control the application of power to theinductive coil 80.

In an alternate embodiment, the plasma is formed using electroncyclotron resonance (ECR). In yet another embodiment, the plasma isformed from the launching of a Helicon wave. In yet another embodiment,the plasma is formed from a propagating surface wave.

It should be understood that various modifications and variations of thepresent invention may be employed in practicing the invention. It istherefore to be understood that, within the scope of the appendedclaims, the invention may be practiced otherwise than as specificallydescribed herein.

1. A method of processing a copper-containing layer in a plasma etcher,the method comprising: providing a copper-containing layer overlying asubstrate; introducing a process gas; forming a plasma from said processgas; introducing from a common source an aluminum-containing materialinto the plasma etcher from an ex situ commercially available gaseoussource of aluminum by separate injection from the common source of thealuminum-containing material both into a plasma processing regionproximate the substrate and into an exterior region removed from thesubstrate; and etching said copper-containing layer by exposing saidcopper-containing layer to said plasma and said aluminum-containingmaterial, wherein said process gas reacts with said copper-containinglayer.
 2. The method according to claim 1, wherein saidaluminum-containing material comprises an amine alane gas.
 3. The methodaccording to claim 1, wherein said aluminum-containing materialscomprises a trialkyl aluminum gas.
 4. The method according to claim 1,wherein said aluminum-containing materials comprises a dialkyl aluminumhydride gas.
 5. The method according to claim 1 wherein said substrateis maintained at a temperature below 200° C.
 6. The method according toclaim 1 wherein said substrate is maintained at a temperature below 150°C.
 7. The method according to claim 1 wherein said substrate ismaintained at a temperature below 100° C.
 8. The method according toclaim 1, wherein said copper-containing layer comprises at least one ofa pure Cu layer and an Al—Cu alloy.
 9. The method according to claim 1,wherein said process gas comprises a chlorine-containing gas.
 10. Themethod according to claim 9, wherein said chlorine-containing gascomprises at least one of Cl₂, HCl, BCl₃, SiCl₄, CHCl₃, CCl₄, andaluminum chloride.
 11. The method according to claim 1, wherein saidprocess gas further comprises an inert gas.
 12. The method according toclaim 11, wherein said inert gas comprises at, least one of argon,helium, krypton, xenon, and nitrogen.
 13. The method according to claim1, wherein said aluminum-containing material comprises an aluminumhalide gas.
 14. The method according to claim 13, further comprisingintroducing at least one of argon, helium, krypton, xenon, and nitrogen.15. The method according to claim 13, wherein a flowrate of the aluminumhalide is less than 1000 sccm.
 16. The method according to claim 13,wherein the aluminum halide gas comprises an aluminum halide gas inplasma form.
 17. The method according to claim 13, wherein said aluminumhalide gas comprises an aluminum chloride gas.
 18. The method accordingto claim 17, wherein a flowrate of the aluminum chloride is less than1000 sccm.
 19. The method according to claim 17, wherein the step ofetching comprises etching said copper-containing layer using thealuminum chloride gas in gaseous form.